A rubber composition usable as a raw material of a rubber product is generally produced through the following three processes:
“Masticating Process”
A process of using a mixing/dispersing device to knead a rubber component singly, or a rubber component containing only a filler such as carbon black and not containing any other blending component. Through the masticating process, rubber molecular chains in the rubber component are cleaved and simultaneously the lengths of the molecular chains are made uniform. Additionally, the rubber component is decreased in elasticity to be heightened in plasticity. Thus, the rubber component is good in workability for subsequent rubber kneading.
“Kneading Process”
A process of charging stearic acid, zinc flower, an anti-aging agent, and the like other than any vulcanization-related blending agent into the rubber component yielded after the masticating process; charging a filler further thereinto if no filler is blended in the masticating process; and then using a mixing/dispersing device to knead the resultant blend. In the kneading process, these blending agents are mixed with the rubber component, thereby producing, for example, the following advantageous effects: a rubber product after vulcanization is heightened in strength; the rubber is made good in workability for kneading; and the rubber is prevented from being deteriorated by radicals generated by the cleavage of molecular chains of the rubber.
“Vulcanization-Related Blending Agent Kneading Process”
Vulcanization-related blending agents, such as sulfur or any other vulcanizing agent, and a vulcanization promoter, are charged into the rubber composition yielded after the kneading process, and then the whole is kneaded. After the vulcanization-related blending agent kneading process, the resultant rubber composition is heated to a predetermined temperature or higher. In this case, the vulcanizing agent in the rubber composition reacts with molecules of the rubber so that a crosslinkage structure is formed between the rubber molecules. Thus, the molecules come to have a three-dimensional network structure to give rubber elasticity.
In general, these three processes are not continuously performed. In many cases, for example, the rubber component yielded after the masticating process is once discharged from the mixing/dispersing device, and again charged into another mixing/dispersing device, and subsequently the kneading process is performed. However, useless energy is consumed for kneading again the rubber discharged once after the masticating process and then cooled to a temperature near room temperature while the rubber is heated in the kneading process. Furthermore, when the rubber component in the state of being once cooled is kneaded while mechanical energy is given thereto, a decrease in the rubber viscosity by heating is not made sufficient. Thus, a high shearing force is given to at least a part of the rubber component. As a result, the polymer molecules are excessively cleaved to cause, for example, a problem of lowering a vulcanized rubber to be finally obtained in tensile stress and the like.
Patent Document 1 listed below describes a method for producing a rubber composition, in which a first kneader is used to masticate a master batch containing a rubber component and a filler, and then a second kneader is used to mix blending agents further with the masticated master batch. However, this production method cannot solve the above-mentioned problems.
Patent Document 2 listed below describes a method for producing a rubber composition, in which a rubber masticating process is contrived. However, this production method is equivalent to the prior art in blending agent-kneading process. Thus, this method cannot solve the above-mentioned problems, either.